This invention relates to a laser beam scanning mechanism in which, in the operation of a laser knife under a microscope, a joystick is operated to change the laser irradiation position.
In a surgical operation using a laser knife, the operator shifts the point irradiated with the laser beam by operating a joystick back and forth or right and left while observing the vital tissues through a microscope.
The laser irradiation position is determined by the inclination angle of a mirror. A mechanism for controlling the motion of the stick to control the inclination of the mirror has been disclosed by the specifications of Japanese Patent Application Laid Open Nos. 111295/1977 and 106144/1980, for example.
In the former mechanism, a mirror for reflecting the laser beam is supported by a horizontal shaft, and a stand including the horizontal shaft is supported by a vertical shaft. Servo motors are coupled to the horizontal shaft and the vertical shaft, respectively, so that the mirror is turnable about both the horizontal shaft and the vertical shaft. The horizontal movement and the lateral movement of the joystick operate respective potentiometers. The rotational displacements of the servo motors are detected, to change the resistance of other potentiometers. The two sets of potentiometers form two bridge circuits, so that the servo motors are turned in one direction or the other in response to the movements of the stick, to thereby change the inclination angle of the mirror.
In such an electrical scanning mechanism, two servo motors must be set in the housing of the microscope. Furthermore, since two orthogonal movements are required, two rotary shafts must be provided. Thus, the former mechanism is disadvantageous in that it is intricate in construction and accordingly costly.
On the other hand, the mechanism disclosed by Japanese Patent Application Laid-Open No. 106144/1980 is purely mechanical, employing a gimbal mechanism.
This mechanism is similar to the above-described mechanism in that the mirror is supported by a horizontal shaft, and a U-shaped stand including the horizontal shaft is supported by a vertical shaft, so that the mirror can turn about these shafts. However, it should be noted that, in this mechanism, instead of the servo motors and the potentiometers, several levers are used in combination so that the horizontal and vertical motions of the joystick are transformed into rotational displacements around the vertical and horizontal shafts, respectively.
Such a gimbal mechanism needs no servo motor; however, it requires two orthogonal rotary shafts and several levers in combination. Therefore, the housing of the microscope is necessarily bulky.
It is impossible to transmit the horizontal and vertical movements of the joystick to two rotary shafts through the same number of levers. Therefore, it is difficult to determine the speed-up ratio and the speed-down ratio of the "lever", to thereby make the horizontal and vertical movements coincident with the rates of rotational displacement of the mirror in the horizontal and vertical directions, respectively.
In order to operate orthogonal levers, a pin and an elongated hole must be employed in combination. In this combination, backlash is taken into account, and accordingly, the levers are subject to play. Since the number of levers in the vertical direction is different from that in the horizontal direction, accordingly the amount of play in the vertical direction is different from that in the horizontal direction.
Furthermore, the mechanism suffers from a difficulty in that, although two independent "lever" systems are required in order to transmit the horizontal and vertical movements to the mirror, the inclination of the mirror is not made independent. For instance, when the rotational displacements of the mirror about the horizontal shaft are different, and the stick is moved to the right and to the left with one and the same amplitude, the amounts of the displacement of the mirror about the vertical shaft are different from one another.
The most serious drawback is that, as the mirror is turned around the horizontal and vertical shafts through levers having a large amount of play, it is impossible to finely displace the mirror.
During a surgical operation under microscope, it is essential to finely shift the laser beam irradiation position. When the angle of inclination of the mirror is changed, the laser beam reflected by the mirror is deflected through an angle which is twice the change in the inclination angle of the mirror. The irradiation position is shifted by a value which is obtained by multiplying the angle of deflection by the distance between the mirror and the tissue being operated upon.
Thus, in order to finely control the laser beam irradiation position, it is essential that the inclination angle be finely controlled.
In the gimbal mechanism using the levers, it is difficult to sufficiently reduce the movement of the joystick to thereby change the angle of inclination of the mirror.
As is apparent from the above description, in the conventional mechanism, the reflecting mirror is rotatably supported by two orthogonal axes. Therefore, it has been impossible to reduce the size of the microscope's bulky housing.
In the case of a purely mechanical transmission mechanism, play takes place at all times, and it is difficult to finely adjust the inclination angle of the mirror. On the other hand, the electrical transmission mechanism suffers from the drawback that the microscope's housing is bulky and the resulting device has low operability.